The early discovery of hidden defects in parts and products is of increasing concern to manufacturers as they strive to obtain superior product quality. Particularly, there is a need for the early discovery of defects which could remain latent, or undiscovered, for an indeterminate time.
Thermography, or thermal analysis, has attracted considerable recent attention as one way of discovering such defects. All objects "glow" from thermal radiation with an intensity and "color" which is dependent upon the temperature. At room temperature this "color" is within a range known as infrared and cannot be seen with the unaided eye. At extreme temperatures an object will glow visibly as in the case of iron heated in a fire. This property can be used to measure the temperature of a surface without need for any kind of contact. Any of several types of equipment can convert this temperature information into a black and white or color image that represents the temperatures within the scene. Such equipment can be called a "thermal imager" and can be used to study non-visible properties of electronic assemblies in the hope of locating defective devices.
It has long been known that patterns of heating effects (e.g., patterns of the infrared glow) in a product may be affected by a latent defect; but the heating effect may not be readily detectable for some types of defects. Particularly in bipolar semiconductor circuits, prior thermal analysis ("thermography") techniques have been only marginally effective in locating defects, except in certain limited situations.
Frequently, the analysis techniques employed with such equipment involve elevating the temperature of the object for at least one of several images. Then images obtained under different conditions are compared in an attempt to remove everything from the image which is normal and leave only the image features that relate to the defect. In the case of semiconductor circuits or components, the different conditions can be the normal powered state and normal non-powered state.
One of these prior analysis techniques is known as image subtraction. Generally, in this technique an image, comprising a regular array of values representing infrared radiation, is obtained from a reference sample, which is a high-quality sample of the product, and is subsequently subtracted from a similar image obtained from a test sample, which is a sample, of unknown quality, of the product. The purpose is to remove features from the difference image which are known to be normal, so as to increase the likelihood that any residue in the difference image is indicative of a defect in the product. Available thermal analysis techniques use image subtraction in one form or another. For example, see the description in the article by C. G. Masi, "Finding Board Faults With Thermal Imaging", TEST AND MEASUREMENT WORLD, March., 1989 pp. 100, 111, 112. The image subtraction technique in this article, as described in connection with a circuit board, starts with the board in a known thermal state (e.g., the entire board at 22.degree. C.). The test operators then apply a given power source to the board and monitor changes in the thermogram as the board heats up to operating temperature.
The basic phenomenon employed in image subtraction thermography for such products is black-body radiation from ohmic heating of current-carrying traces and components, as explained in the article by C. G. Masi, "What Can Thermal Imaging Do For You?" TEST AND MEASUREMENT WORLD, May, 1988. It is also known, however, that image subtraction thermography can be applied to non-current-carrying products, to the extent such products can be subjected to controlled thermal changes.
The monitoring of thermal changes by image subtraction is based on the premise that the thermal changes for a non-defective product should be different from the thermal changes for a defective product. Thus, if the thermogram for a known non-defective product (sample) is subtracted from the thermogram for a product ( or sample) being tested, the differences, if any, are hoped to be indicative of a defect. Conversely, if a hidden defect does exist in a sample, it is hoped that it will produce a thermogram which is different from the thermogram of the non-defective sample. The greatest successes in the prior art techniques have been for products which produce relatively little heat. However, products such as transistor-transistor-logic circuits which produce much heat have yielded marginal success in diagnostic testing using prior art techniques. The problem appears to be that the variability of heating among non-defective (normal) samples can be much larger than the effect upon heating produced by a sample having a subtle defect. This tendency makes such defect difficult, if not impossible, to detect by previously known image subtraction techniques.